First of yes i know this won't happen -

Okay I have seen people complain about the Broken Steel expansion and I get what people are upset about. It really does invalidate some decisions but that doesn't mean every after release DLC has to do this.

So here is my idea on how they can do it right, even though they won't do it at all.

After the battle of hover damn simple recognise whoever is in control with their military presence. Then simply say a new threat has emerged, Vault 22 residence have returned, Yes man has mutanied or downloaded himself to another council whatever. The threat can be whatever. This works because it
1) Doesn't invalidate any endings
2) Still continues while representing the players choices.

The Faction head (which will probably be you, House lets you handle a lot of day to day, you are the NCR commander in the area, you are Cieser chosen governor and Independent well you are running things anyway.) you have to deal with this new threat which will take you to a new area like Broken Steal without screwing your choices over (I can't stress this enough).

Then you simply go about dealing with this threat.

Now this could be as simple as going to a new area or have these creatures, faction, whatever start effecting different areas of the map which you can help defend, recapture, ect. Or it could be as simple as just a new area.

Something that should be added in my opinion is some new creatures to the wasteland like many people have been asking for.

Anyway just my idea on how they could do an after Main Quest expansion without pissing off half the fan base.

Sounds good but won't happen

Or they could always just avoid doing one what so ever, minimizing the threat of offending the fans even more.
Though I wouldn't mind a full sized expansion taking place after the game on a new location. (As having it take place during the main quest would be weird. (That the Courier just decides to skip town for the DLC's is weird enough.))
Might not work though, doesn't the Courier stay in Vegas with the House and Yes Man ending?
But then I don't really think the Courier is needed anyway, why simply avoid the main game like the plague cause of the vast variety of ending sliders by placing it in a new area and having the Courier go there, when they could just create an expansion with a new protagonist all together? (All new weapons, armors, crafting and perks are added to the main game to use as well.)

It just doesn't fit in with The Courier really.

Or a DLC that lets you go to a new area after the main quest, HOWEVER once you finish that DLC the game ends...FOREVAH. But you can free roam in the new area before you finish, maybe add new side-quests and enemies, but not let you go back to the Mojave.

Still seems pointless though.
I mean, if the entire main quest and ending sliders is gonna be avoided then what connection will it have to the main game?
And why do the Courier have to be the protagonist of the DLC?
Doesn't make sense to me.

Who cares if it's set after the main quest, nothing will happen anyway.
It will be like Dead Money or Honest Hearts, a big sidequest in a small new area.
Only difference is that it is locked until we complete the main quest.
And the Courier? Why does he/she have to be there? Couldn't it use a new protagonist?
Just seems so pointless to me.

So why even do a post-game DLC?
Cause that "post game" DLC could just as well be another regular DLC that you don't have to wait until everything else is done to do.

It would be difficult to do even with a full expansion. There are too many factors to deal with.